For a problem-free job, brace thoroughly, add superplasticizer to the mix, and place the concrete in lifts

As a design-build contractor in an area with no shortage of
upscale home buyers, I have quite a few clients who care more
about getting a good product than they do about watching every
dime. As a result, I often have a chance to try out new
products and ideas. One thing I do with that freedom is try to
steer my designs in the energy-efficient and "green" direction,
while meeting the client's other objectives.

On a recent project, I used the Polysteel insulated concrete
form (ICF) system to build the foundations for three connected
buildings. I'm glad I gave this system a chance: The results
were everything I had hoped for, and the few minor glitches we
encountered will be easy to avoid next time around.

Why ICFs?

The high insulation value of an ICF wall wasn't my primary
reason for using the system. The main thing, frankly, was
convenience: ICFs gave me more control over schedule and
quality, and they gave me design flexibility.

In our area, you can wait a long time for a foundation
contractor to fit you into his busy life. With an ICF system,
my crew and I could form the footings and walls ourselves
instead of having to work around a subcontractor's
schedule.

Also, with the time pressure they face, poured-wall
contractors don't always produce accurate work, and anyone who
builds houses knows how much trouble it can be to adapt your
wooden structures to a concrete foundation that is not quite
true or level. By setting our own forms, my carpenters and I
knew that if the basement didn't end up the way we wanted it,
we'd have only ourselves to blame. It wouldn't take us any
longer to place our own forms accurately than it would to level
and square the deck on somebody else's concrete work.

Finally, ICFs made it easy to accomplish my stepped foundation
design. I wanted to preserve a lot of the natural landscape on
the site, and I wanted the building to conform to the contours
of the existing terrain. The excavator I work with is willing
to do things carefully, so instead of just hogging out a giant
pit for the basement and worrying about the landscaping later,
I planned to shape the foundation to the hillside. I ended up
with a section of full 8-foot basement, a section of 4-foot
crawlspace, and a section of 6-foot partial basement. Stacking
4-foot by 16-inch ICF blocks let us easily match the walls with
the stepped footings as we went up the grade.

Energy advantage. Although
construction efficiency was my first consideration on this job,
I don't mean to make light of the energy efficiency of ICFs.
Above grade, ICF walls rate an R-20 or better, and they're
impressively airtight as well. R-20 is heavy insulation for a
basement, and the wall's performance when buffered by earth is
probably quite a bit better than even that rating would
indicate. I'm not sure how much the basement affected the whole
building's heat-loss calculations, but I would say that this
basement feels much more snug, warm, and dry than most
basements I have built.

I would have gone to some trouble to insulate this foundation
even if it had been a conventional basement. But when you stud
out a basement and insulate the cavities, you're stealing
living space; and you sometimes encounter moisture problems,
too. If you put foam on the outside of a conventional concrete
basement, you can have termite and ant problems, and aligning
the siding becomes a concern (typically, you have to cantilever
the sill out over the foam). Either way, there's labor
involved. With ICFs, on the other hand, you're insulating at
the same time you set up your forms.

Cost wasn't really an issue for my clients on this custom job,
so I was expecting changes during construction. And I was
right: We ended up moving some windows on the first and second
floors. Needless to say, I was glad those above-grade walls
were not cast in concrete.

For a fixed design, however, such as a row house, I can
certainly see some advantages to using ICFs for the whole
house. The day may come when I take a chance on building a
whole-ICF home.

Choosing a Manufacturer

This was my first ICF project, and I don't know enough about
the different ICF systems to recommend one over another. Quite
honestly, I picked Polysteel (American Polysteel, Inc.,
800/977-3676, www.polysteel.com) because they had a
distributor not too far away, and because they were the
quickest to supply us with useful information. Their
instruction booklet is clear and comprehensive enough that I
didn't need to call the company for any help or advice during
the project. They also sent us their video, which the crew and
I watched together; that convinced us all that we'd be able to
handle the job without any trouble.

Polysteel also has the advantage of being treated with borates
to resist ant and termite infestation. That saved us from
having to install termite shields between the foundation and
the house frame, and it let us bury the foam and stucco over
the above-grade portion, without any concern that bugs would
cause hidden damage. If you chose a brand with no borate
treatment in the foam, you'd need to think about the insect
protection issues.

I was very happy with the way the Polysteel forms worked. They
were easy to assemble, and the steel fins on the outside face
accepted screws readily, so it was easy to fasten bracing to
the forms and to make splices when we needed to cut a form to
length. Other forms might have other advantages, but all in
all, I was satisfied with my choice.

Stacking the Forms

Polysteel forms are 4 feet long, and they come in 1-foot,
2-foot, and 16-inch heights. The top, bottom, and ends have a
tongue-and-groove profile, so they stack precisely and the
forms lock together to a certain extent. Setting up the forms
is simple. You pour your footing using conventional methods,
and snap lines for your foundation footprint the same way you
would for conventional forms. The first course of forms is
adhered to the footing with a thin, continuous bead of
polyurethane foam. From there on up, setting the forms is
something like building with toy Lego blocks: You just stack
them up, using a little dab of foam at all abutting edges to
"tack" things together. (We cemented all the blocks together
with a continuous bead of foam because we had plenty on hand,
and that worked fine -- just don't use so much foam that it
pushes things out of alignment as it expands.) When you come to
a corner, you use a right-hand or left-hand corner form.

Layout. The footprints of my
three connected buildings had a few jigs and jogs in them, but
they weren't especially complicated, and there were only right
angles -- no odd bends or curves. So I didn't have to learn any
of Polysteel's methods for making mitered corners (although
their handbook provided detailed instructions for that, and it
doesn't look too tough). For the sake of efficiency, I based my
plans as much as possible around the 4-foot and 2-foot modules.
A few odd wall lengths required us to trim forms, and this was
not a big problem, even though we had a minor incident during
the pour when a splice started to give way. For the sake of
simplicity, it's nice to avoid the odd dimensions if you
can.

A more critical layout concern has to do with how you place
your rebar. Every wall gets some amount of reinforcing steel,
and the vertical bars within the wall are tied into short stubs
that are wet-set into the footings. The rebar has to sit in the
middle of the 1-foot-on-center core voids in the waffle forms,
starting with a bar rising up the center of the corner cavity
(see Figure 1). So it's important to know how your foam forms
will lay out, and to mark the footing forms for the proper
rebar locations (or else be ready to measure carefully as you
place your rebar stubs into the footing).

Figure 1.Rebar stubs must be accurately set into
the footings to ensure that the vertical bars are centered
within the core voids. The exterior of the completed wall has
flat, parallel faces, but the waffle-shaped interior reduces
the amount of concrete needed while still providing a
high-strength product.

Placing forms. You stack the
blocks up one course at a time, always beginning in a corner
(Figure 2). If your foundation is the right size, you can go
all the way around the footprint without cutting any forms
until you get back to where you started. But you'll usually
have to cut at least one form at the end of the circuit, and
you may need to cut one in every length of wall as you approach
the corner. In any case, you alternate left and right corner
forms as you go up the wall, and stagger the end joints of the
form blocks so they don't fall out above one another.

Figure 2.Walls are stacked one course at a time;
corners are laid up using alternating left-hand and right-hand
corner blocks. Polyurethane foam is used as "glue" between
trimmed forms that lack the usual tongue-and-groove connection,
but added exterior reinforcement is also needed in those
areas.

Cutting and splicing.
Whenever you cut a form, you're trimming off the
tongue-and-groove connection, and you need to replace it with a
strong splice. Running a bead of polyurethane foam between the
cut forms helps keep them in alignment, but the easiest way to
provide the needed structural strength is to place a small
piece of plywood to span over the joint and screw it to the
nearest steel furring strips on the forms. In our experience,
it's worth taking time with splices, and even providing each
splice with its own brace back against the soil. The only time
we had trouble during our pour was when a splice started to
give way.